Hacking Sena SMH10

I've been riding with Sena SMH10 for over a year and like it a lot. I am not a fan of helmet speakers, and have been using their optional earbuds clamp almost exclusively. I decided to convert the stock helmet speaker clamp to earbuds to use with my other helmet. This clamp has a 3.5mm aux jack for use with audio sources without Bluetooth. My sole source is an Android smartphone, so I've never used the aux jack and it would serve me better re-purposed as an earbuds jack.

This is a factory earbuds clamp disassembled. The audio is fed to the earbuds though a pair of 75 Ohm resistors because earbuds need far less power than the helmet speakers.

I had 51 Ohm resistors handy, they also work. Audio is a bit louder, but not by much.

This how I did it:
To disassemble the clamp, remove two visible screws and two screws under the rubber cushion.

Then slide the microphone boom shroud towards the mike and the clamp will come apart.
I don't need the speakers, so I unsoldered and removed them, however it should be possible to have both speakers and earbuds working at the same time.

Remove the PC board, turn it over and cut four traces with a sharp blade or knife.

Sorry for the bad focus, I think the traces are still visible.

Solder resistors between the jack and "R+" and "L+" points. Probably surface mounted chip resistor will be a bit easier to fit in.

Solder the common wire between the jack and "L-"

Reassemble in the reverse order, correctly positioning the mike boom. (The triangle mark on the mike should point toward your mouth). I removed the speakers and used a "liquid gasket" silicone compound to waterproof the opening for the cables. If you don't want to do it, you can leave the speakers in place or cut their wires flush.

I SO wish Sena would just make helmet clamp with speakers AND the Aux In jack wired as Earbuds Out! But, your pics and directions should make it feasible to modify a stock clamp to do both.

What is really the downside of not putting those resistors in? Just that my ear buds will be louder? Since I can turn down the Vol on the SMH10, I'm a bit unclear on why I would care.

And if I leave the stock speaker wires in there, they would run at the same time, right, whether I have my earbuds plugged in or not, yes?

Now I really want to try this. I have been wanting a clamp that I could use to listen to music even when I'm just on a short trip and don't want to hassle with putting my ear buds in.

So, if I've read all this correctly, to leave the stock speakers intact and functional and just convert the Aux In to Earbud Out, all I have to do is cut those 4 traces and solder in 3 wires, right (or 2 resistors and 1 wire, if I want to knock down the volume on the ear buds)?

Without the resistors the audio levels will be much higher. Usually Sena is loud enough, but i encountered some files, mostly podcasts and audiobooks which are recorded too quiet. Increasing loudness by removing resistors is tempting, but it will make Sena's own system beeps and vocal prompts dangerously loud.
If the speakers are left connected and installed in the helmet, some sound from them will leak through even with insulated earbuds. I don't know how it will affect the listening experience.

Have you seen battery life or earbud volume affected when running both speakers+earbuds simultaneously? I was wondering because I had thoughts to create a drop-in replacement PCB board with a switched headphone jack: normally connected to the speakers, and re-reouting the signal to the earbuds only when plugged in.

But it might not be worth it if there is little difference to batt or vol.
Thoughts?

Sorry, I don't know how running both earbuds and speakers affects the battery. I'm trying to prevent full discharge cycles and recharge my battery well before it gets depleted. Anyway, the difference is not that big and it will be difficult to measure just by riding. And Sena doesn't say that the running time with the earbuds option is better than with the speakers. In my experience, battery is good enough for a full day of riding, so it has to be charged in the evening regardless of the remaining charge.

Thanks Genka,,
I've been pleased with the talk and battery time of the units fro a few years now, and dont expect this to appreciable affect the battery's up time.

But now that I have them in and tested, I am disappointed by the sound quality to the earbuds. Audio seems to be a reduced dynamic range mono and very flat sounding, almost AM radio. The same earbuds directly plugged in to my android sound quite good. I double checked my soldering and resistor connections. Essentially, music and voice quality are similar to the original speakers :-(.

Does anyone know if this a a Sena-fidelity issue or related to the hack? I've paired other A2DP bluetooth headsets with excellent results. I have no experience with the official earbud mounting plate for comparison.

ago I cut the wires going to the speakers and added a jack. If you wanted speakers or earbuds you just plug in what you want. Does not overdrive and works great with either or both.

Click to expand...

I would think the original clamp kit without the 75 ohm resistors could really blow out your earbuds unless you always remember to turn them down.. i mean you have the speakers on then when you unplug them and plug in your earbuds it would be very loud in your earbuds and could blow them out... You haven't had any problems??

I've emailed back and forth with genka and I've done the mod myself and here is the collective update notes:

- The way genka did the wiring (in the OP), the L and R channels are reversed.

- Also, the stereo separation between L & R is very poor.

- If you leave out the resistor, then all you have to do is turn down the volume when using ear buds and it will not be too loud - neither your music nor the Sena prompts. But, as noted, yes, if you use the built in speakers and have them turned way up, then plug in ear buds, they will be VERY loud until you turn the Sena volume down.

- In order to fix the L&R reversal, just swap the routing of the two wires that go to the L+ and R+ terminals. I think it makes it easier as the R+ becomes very short and no wires have to cross over each other.

- In order to improve the stereo separation, add a 4th wire that connects from R- to either L- or the Ground terminal (which L- is hooked to).

I did the initial mod, with no resistor and L&R preserved, and it works fine. I could not tell there was a problem with the stereo separation until I played a test MP3 that actually went full L, then full R. genka said the same thing. He had never realized there was a problem there until I reported it and he tested it himself.

I have not done the additional mod of adding a ground to R-. genka did it and reported the stereo separation to be greatly improved.

Here are a couple of pics of my version of the mod. No need to point out that I am a complete HACK with a soldering iron :

Have any of you identified what each of the 12 pins on the base unit are used for? There are 12 and if I allow 4 for the mic, L-spkr, r-spkr and gnd PLUS 3 for the 3.5mm stereo input jack, that only makes 7. Just trying to understand if there are 5 spares or there are unique pairs used for the mic and each speaker or some other explanation. One of my pins has fallen out but so far all I've confirmed is my cellphone pairing still works OK. IF the pin that fell out is spare, I will be very happy. Cheers Chris

You might give Marilyn at Plug Up a call. A friend had her replace the factory speakers with a headphone jack so he could use his own headphones and he reports great results. She has done a bit of work for me...repairing a pair of custom ear phones...reasonably priced and quick.

You might give Marilyn at Plug Up a call. A friend had her replace the factory speakers with a headphone jack so he could use his own headphones and he reports great results. She has done a bit of work for me...repairing a pair of custom ear phones...reasonably priced and quick.

I have the earbud baseplate. A buddy and I both broke ours on the same trip: where the flexible rubber mold meets the hard plastic body of the baseplate. So I foresee this eventually being a problem for most users with this type of baseplate.

Unfortunately, I don't have a pic prior to disassembly. Here, I dissected the soft rubber cover to see what I was dealing with.

My buddy approached it a bit differently by replacing the output and relocating it to the bottom of the baseplate. He did an excellent repair but I was stuck on the same idea that genka had.

I simply reused the broken wires

and jumpered the normal output to the appropriate sides on the fixed audio jack. However, I did replace the ground with a shielded wire.

Note that the right channel is not visible because it's under the ground wire. And I didn't have to add the 75 &#937; resistors because they were already there.

Fortunately, I found this thread before attacking. Already knowing which traces to cut (to keep from shorting the aux in with the earbud out) saved me some time.

My observations of repurposing the aux in as the earbud output:
1. It's difficult to locate the new output in everyday use; with the baseplate mounted to the side of your head and motorcycle gloves on.
2. The microphone wire needs to be routed with attention so that it does not get in the way while plugging in the headphones.

StuartV pointed me on 2 problems with the mod. The first one is fairly trivial- I managed to switch L and R channels. The fix as as simple as wearing the earbuds appropriately. Also this problem is unnoticeable because of the second one.
The second problem is serious- the stereo signal separation is almost nonexistent. The reason for this is that Sena uses differential amplifiers for the helmet speakers, and both "+" and "-" terminals have signals on it. It works fine for the speakers, because they have independent wires. Earbuds have one of the wires joined together. Normally it wouldn't be a problem, but when this joined common wire is connected to joined "-" wires of the amplifier, the severe cross channel bleeding occurs. Let me make it clear- this is not a problem with my mod. Sena factory earbud clamp suffers from the same issue. This is a design flaw.
There is one way to work around the flaw- to connect the common earbud wire to the ground. It fixes the stereo problem, but introduces a new one- an interference from the digital section of the main unit can now be heard as a buzzing sound in the earbuds. It is very noticeable during quiet moments, but is completely masked by a loud music or wind noise while in motion. I'd say if you listen to a lot of music this fix pays off, but if you are more interested in intercom, phone, audio books or talk show podcasts, you'll be better of with the degraded stereo sound without the background noise. It is also possible that the noise level may very between the main units, maybe mine is particularly noisy.

To use the common ground for better stereo it is necessary to remove the jumper from the "L-" point and to resolder a trace which was cut. If you start with an unmolested clamp, you just cut 3 traces instead of 4.

The restored trace is on the left

The final layout for improved stereo

This is a track you can play on your smartphone to check stereo separation and alignment
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What Sena did with their earbud clamp is such a glaring design error I am not sure how it could slipped past their QA. I urge everyone to contact Sena and request a fix.